i am looking for help with a domain error.

when i visit my domain on http://mydomain.com i am forwarded to the following

http://www.mydomain.com/index.php?www

i dont know how this is happening and i dont want it, this is now starting to cause me problems as my domain is not recognised when i am trying to configure settings from an ad network.

anybody know how this forwarding takes place and how i can stop it?